Diffusion-Controlled Spherulite Growth in Obsidian Inferred from H2O Concentration Profiles
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Contrib Mineral Petrol DOI 10.1007/s00410-008-0327-8 ORIGINAL PAPER Diffusion-controlled spherulite growth in obsidian inferred from H2O concentration profiles Jim Watkins Æ Michael Manga Æ Christian Huber Æ Michael Martin Received: 3 November 2007 / Accepted: 8 July 2008 Ó Springer-Verlag 2008 Abstract Spherulites are spherical clusters of radiating Keywords Spherulites Á Obsidian Á FTIR Á crystals that occur naturally in rhyolitic obsidian. The Advection–diffusion growth of spherulites requires diffusion and uptake of crystal forming components from the host rhyolite melt or glass, and rejection of non-crystal forming components Introduction from the crystallizing region. Water concentration profiles measured by synchrotron-source Fourier transform spec- Spherulites are polycrystalline solids that develop under troscopy reveal that water is expelled into the surrounding highly non-equilibrium conditions in liquids (Keith and matrix during spherulite growth, and that it diffuses out- Padden 1963). Natural spherulites are commonly found in ward ahead of the advancing crystalline front. We compare rhyolitic obsidian and have evoked the curiosity of petrol- these profiles to models of water diffusion in rhyolite to ogists for more than a century (e.g., Judd 1888; Cross 1891). estimate timescales for spherulite growth. Using a diffu- Over the past several decades, numerous studies on spher- sion-controlled growth law, we find that spherulites can ulite morphology (Keith and Padden 1964a; Lofgren grow on the order of days to months at temperatures above 1971a), kinetics of spherulite growth (Keith and Padden the glass transition. The diffusion-controlled growth law 1964b; Lofgren 1971b), disequilibrium crystal growth rates also accounts for spherulite size distribution, spherulite and textures (Fenn 1977; Swanson 1977), and field obser- growth below the glass transition, and why spherulitic vations (Mittwede 1988; Swanson et al. 1989; Manley and glasses are not completely devitrified. Fink 1987; Manley 1992; Davis and McPhie 1996; Mac- Arthur et al. 1998, and many others) have shed light on the conditions and processes behind spherulite nucleation and Communicated by J. Blundy. growth. Despite the long-standing interest, several funda- mental questions persist. For example: On what timescales J. Watkins (&) Á M. Manga Á C. Huber do natural spherulites form? At what degree of undercooling Department of Earth and Planetary Science, do spherulites begin to grow? Can spherulites grow below University of California, Berkeley, CA, USA the glass transition? Why doesn’t spherulite growth lead to e-mail: [email protected] runaway heating? And, why are spherulites spherical? In M. Manga this paper, we revisit these questions and contribute a new e-mail: [email protected] quantitative approach for estimating spherulite growth rates. C. Huber e-mail: [email protected] Qualitative description of spherulite growth M. Martin Advanced Light Source, Spherulites in rhyolitic obsidian are composed primarily of Lawrence Berkeley National Laboratory, Berkeley, CA, USA alkali feldspar and SiO2 polymorphs. Spherulites are often e-mail: [email protected] mm-scale, but are known to range from submicroscopic to 123 Contrib Mineral Petrol m-scale (e.g., Smith et al. 2001). They may be randomly Tequila volcanic field, western Mexico. Lavas that range in distributed and completely isolated, or nucleate preferen- composition from basalt to rhyolite surround an andesite tially to form clusters or trains (e.g., Davis and McPhie stratovolcano (Volca´n Tequila) that formed about 200 kyr 1996). Within a deposit, spherulitic textures and relation- ago (Lewis-Kenedi et al. 2005). The rhyolites and obsidian ships may vary widely owing to local differences in domes are among the earliest erupted units, ranging in age emplacement temperature, flow depth, cooling rate, and from 0.23 to 1.0 Ma (Lewis-Kenedi et al. 2005). surface hydrology (Manley 1992; MacArthur et al. 1998). Figure 1 is a photograph of the obsidian hand sample Despite these complexities, the fact that spherulites occur used in this study. The matrix is a dark, coherent glass that almost exclusively in glasses seems to require conditions has not undergone post-magmatic hydration. It contains that yield slow nucleation rates yet relatively rapid crystal some visible but subtle ‘‘flow bands’’ reflecting shear flow growth rates. of magmatic parcels containing variable microlite content As a melt cools below the liquidus temperature (Teq) for (Gonnermann and Manga 2003). Isolated, light grey a given crystalline phase, an energy barrier must be over- spherulites cross-cut flow bands and exhibit a range of come in order for the new phase to nucleate and grow. Just sizes that are distributed homogeneously (Fig. 1). below Teq, this energy barrier is finite and decreases with Individual spherulites are host to three crystalline pha- decreasing temperature. A certain amount of undercooling ses: 55–65% (by volume) feldspar fibers, *30–40% oblate is always necessary to nucleate a solid phase (Carmichael cristobalite masses, and \1% Fe- and Ti-oxides. The et al. 1974), where the degree of undercooling (DT) is the feldspar fibers are arranged in a radial habit about a central difference between the equilibrium temperature Teq and the nucleus and are enclosed by the cristobalite masses. X-ray actual magmatic temperature T. In siliceous lavas, atomic diffraction patterns indicate the feldspar is some form of mobility is sluggish and large DT can be achieved prior to alkali feldspar, but an exact determination could not be nucleation. Once nucleation does occur, crystal growth made. There is also a significant amount of void space due proceeds and crystal growth rates are dictated by the to the *10% volume contraction associated with crystal- magnitude of DT. lization, which we neglect in our calculations. Large undercoolings promote rapid crystallization, but are also associated with lower magmatic temperatures and slower elemental diffusivities. For crystals growing from a Sample preparation and measurements melt or glass of different composition, crystal growth rates are limited by the ability of melt components to diffuse Four rectangular billets (*21 mm 9 38 mm) of spheru- towards or away from the crystallizing front. In the case of litic obsidian were cut from a hand sample (Fig. 1)tobe spherulites, it is has been shown that certain major- and trace- prepared for FTIR analysis. Each billet was ground down elements are rejected by crystallizing phases and concen- until at least one spherulite was sectioned through its trated at the spherulite–glass interface (Smith et al. 2001). center. The surface with the spherulite was polished to Since the mineral phases that compose rhyolitic *0.25 lm. The polished surface was mounted on a glass spherulites are nominally anhydrous, water is also rejected slide with epoxy and the billet was cut to a thickness of from the crystalline region. Among incompatible melt *300 lm. The grinding and polishing procedure was species, water is perhaps the most extensively studied repeated on the opposite surface to obtain a doubly pol- because of its pronounced effects on magma viscosity and ished wafer. Each of the samples was of uniform thickness major-element diffusion coefficients. As a result, methods (±\5% as determined by a digital micrometer with a for determining water concentrations in rhyolitic glasses precision of ±2 lm) so that infrared absorption data could have been developed over the past 25 years (e.g., Stolper be compared throughout the sample. Polished surfaces 1982; Newman et al. 1986, Zhang et al. 1997) using Fou- were necessary for minimizing infrared scattering and rier transform infrared spectroscopy (FTIR). The goal of obtaining quality absorbance data. our measurements is to use FTIR to resolve water con- Water concentration measurements were made by syn- centration gradients at the spherulite–glass interface with chrotron-source FTIR at the Advanced Light Source at high spatial resolution (see Castro et al. 2005) to better Lawrence Berkeley National Laboratory. The instrument understand the growth history of spherulites. used was a Nicolet 760 FTIR spectrometer interfaced with a Spectra-Tech Nic-Plan IR microscope capable of spot sizes of 3–10 lm (Martin and McKinney 1998). Transects Sample descriptions were oriented perpendicular to spherulite rims and mea- surements were made on the surrounding glassy matrix. The spherulites analyzed in this study are from a hand Total water concentrations were determined from the sample of a rhyolitic vitrophyre that lies in the Quaternary intensity of the 3,570 cm-1 peak (Newman et al. 1986), 123 Contrib Mineral Petrol Fig. 1 a Photograph of a spherulitic obsidian from b) Tequila volcano. Spherulites ranging in size from 1.0 to 8.0 mm in diameter are a) distributed homogeneously throughout. b Cathodilluminescence image showing fibrous alkali-feldspar crystals enclosed by oblate 100 µm cristobalite masses within an individual spherulite. c Thin section image showing spherulites cross-cutting subtle flow bands c) 5 mm 5 cm which reflects the abundance of hydrous species (XOH) elements could not be resolved quantitatively within including molecular water (HOH). Hereafter, we refer to detection limits. However, qualitative variations are all hydrous species collectively as ‘‘water’’. Analytical resolved through detailed WDS X-ray mapping for three error is estimated to be ±0.005 wt% based on repeated major elements taken on